Semiconductor wafers generally undergo several processes, such as a film forming, an oxidation processing and a diffusion processing, during the manufacturing of semiconductor integrated circuits. Vertical processing units are often used for processing, as they can process many wafers at a time.
Vertical processing units comprise a cylindrical processing chamber with a ceiling. The chamber has an opening inside of the bottom surface, through which multiple semiconductor wafers held by a wafer-boat are loaded into the processing chamber. The processing chamber is then tightly closed, and the wafers undergo processing, e.g. are heated, exposed to certain gases and then subjected to a further heating.
The atmosphere in the processing chamber may be at a vacuum or at substantially the atmospheric pressure, depending on the type of process to be conducted. A variety of gases are also used for processing. If the processing pressure in the processing chamber is reduced, the processing gas in the chamber seldom leaks out of the chamber because the gas is sucked out with a vacuum pump. On the other hand, when the processing pressure in the processing chamber is maintained near the atmospheric pressure, the processing gas may leak out of the chamber because the gas in the chamber is not sucked out so strongly. In order to prevent this, a reliable scaling structure has to be provided.
In particular, when the processing pressure is at substantially the atmospheric pressure and the processing gas comprises a corrosive gas such as hydrochloric acid (HCl), for example in the oxidation process, the sealing structure for the processing chamber has to be highly sophisticated.
The sealing structure in a conventional processing unit is as follows. FIG. 11 is a schematic view of a conventional vertical processing unit. FIG. 12 is a plan view of a cap in the processing unlit shown in FIG. 11. FIG. 13 is an enlarged partial view of a sealing structure between the bottom surface of the processing chamber and the cap.
As shown in FIG. 11, the processing chamber 2 made of quartz is a cylinder with the top end closed by a ceiling and the bottom end left open. A flange 4 for connecting with the cap 6 is formed around the bottom circumference of the cylinder.
The bottom of the processing chamber 2 is sealed by the disk-shaped cap 6 (see FIG. 12) made of quartz, which can be moved vertically by an elevating mechanism (not shown). In FIG. 11, the cap 6 is positioned a short distance below the bottom of the processing chamber 2. A quartz wafer-boat 8 is mounted on a thermal tube 10, which is mounted on the cap 6. The wafer-boat holds wafers W in a number of stages, each spaced at a specific pitch. The wafer-boat 8 can be loaded into and unloaded from the processing chamber 2 by raising and lowering the cap 6.
An abutting-surface 12 on the bottom surface of the flange 4 tightly connects with an abutting-surface 14 of the cap 6 when the wafers are loaded into the processing chamber 2. The abutting-surface 14 forms an annular protrusion which protrudes above the upper surface of the cap 6. As shown in FIG. 13, a header 16 for supplying an inert gas runs along an intermediate portion of the abutting-surface 12 of the flange 4. The header 16 has a concave section. An inert gas such as nitrogen (N.sub.2) gas is supplied into the header 16 at a certain pressure, so that the processing gas (the environmental gas) in the processing chamber 2 is prevented from leaking out. This is effective even when the plane contact (plane abutting) between both the abutting-surfaces 12, 14 is not uniform.
In the above described structure, both the abutting-surface 12 on the flange 4 of the processing chamber 2 and the abutting-surface 14 of the cap 6 have to be manufactured with a high degree of precision for achieving a highly sophisticated sealing. It is difficult to form a flat surface of quartz over an annular plane area with a high precision because quartz has a very poor workability. Thus, any imperfections in the annular abutting-surfaces 12, 14 result in a less-than-perfect seal. If this happens, the processing gas (e.g. corrosive gas) in the processing chamber 2 may leak out of the processing chamber 2, even if nitrogen (N.sub.2) gas is supplied into the header 16 to bring the interface between both the abutting-surfaces 12, 14 at a positive pressure.
Even if both of the abutting-surfaces 12, 14 are manufactured with a high precision, slight discrepancies in the assembly process may lead to the less-than-perfect contact and result in the processing gas leaking out of the processing chamber. In addition, it takes a long time to manufacture broad abutting-surfaces such as those in 12 and 14 with a high degree of precision.
To solve the above problems, a large amount of compressed inert gas could be pumped into the processing chamber. But this would also increase the amount of inert gas, so that the processes on the wafers may be unevenly conducted among the wafers or over each surface of each wafer.